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.TH MSGRCV 2 "May 19, 1999"
.SH NAME
msgrcv \- message receive operation
.SH SYNOPSIS
.LP
.nf
#include <sys/msg.h>

\fBssize_t\fR \fBmsgrcv\fR(\fBint\fR \fImsqid\fR, \fBvoid *\fR\fImsgp\fR, \fBsize_t\fR \fImsgsz\fR,
     \fBlong int\fR \fImsgtyp\fR, \fBint\fR \fImsgflg\fR);
.fi

.SH DESCRIPTION
.sp
.LP
The \fBmsgrcv()\fR function reads a message from the queue associated with the
message queue identifier specified by \fImsqid\fR and places it in the
user-defined buffer pointed to by \fImsgp\fR.
.sp
.LP
The \fImsgp\fR argument points to a user-defined buffer that must contain first
a field of type \fBlong int\fR that will specify the type of the message, and
then a data portion that will hold the data bytes of the message. The structure
below is an example of what this user-defined buffer might look like:
.sp
.in +2
.nf
struct mymsg {
        long int    mtype;     /* message type */
        char        mtext[1];  /* message text */
}
.fi
.in -2

.sp
.LP
The \fBmtype\fR member is the received message's type as specified by the
sending process.
.sp
.LP
The \fBmtext\fR member is the text of the message.
.sp
.LP
The  \fImsgsz\fR argument specifies the size in bytes of \fBmtext\fR. The
received message is truncated to \fImsgsz\fR bytes if it is larger than
\fImsgsz\fR and (\fImsgflg\fR\fB&MSG_NOERROR\fR) is non-zero. The truncated
part of the message is lost and no indication of the truncation is given to the
calling process.
.sp
.LP
The \fImsgtyp\fR argument specifies the type of message requested as follows:
.RS +4
.TP
.ie t \(bu
.el o
If \fImsgtyp\fR is 0, the first message on the queue is received.
.RE
.RS +4
.TP
.ie t \(bu
.el o
If \fImsgtyp\fR is greater than 0, the first message of type \fImsgtyp\fR is
received.
.RE
.RS +4
.TP
.ie t \(bu
.el o
If \fImsgtyp\fR is less than 0, the first message of the lowest type that is
less than or equal to the absolute value of \fImsgtyp\fR is received.
.RE
.sp
.LP
The \fImsgflg\fR argument specifies which of the following actions is to be
taken if a message of the desired type is not on the queue:
.RS +4
.TP
.ie t \(bu
.el o
If (\fImsgflg\fR\fB&IPC_NOWAIT\fR) is non-zero, the calling process will return
immediately with a return value of \(mi1 and \fBerrno\fR set to \fBENOMSG\fR.
.RE
.RS +4
.TP
.ie t \(bu
.el o
If (\fImsgflg\fR\fB&IPC_NOWAIT\fR) is 0, the calling process will suspend
execution until one of the following occurs:
.RS +4
.TP
.ie t \(bu
.el o
A message of the desired type is placed on the queue.
.RE
.RS +4
.TP
.ie t \(bu
.el o
The message queue identifier \fImsqid\fR is removed from the system (see
\fBmsgctl\fR(2)); when this occurs, \fBerrno\fR is set equal to \fBEIDRM\fR and
\fB\(mi1\fR is returned.
.RE
.RS +4
.TP
.ie t \(bu
.el o
The calling process receives a signal that is to be caught; in this case a
message is not received and the calling process resumes execution in the manner
prescribed in \fBsigaction\fR(2).
.RE
.RE
.sp
.LP
Upon successful completion, the following actions are taken with respect to the
data structure associated with \fImsqid\fR (see \fBIntro\fR(2)):
.RS +4
.TP
.ie t \(bu
.el o
\fBmsg_qnum\fR is decremented by 1.
.RE
.RS +4
.TP
.ie t \(bu
.el o
\fBmsg_lrpid\fR is set equal to the process \fBID\fR of the calling process.
.RE
.RS +4
.TP
.ie t \(bu
.el o
\fBmsg_rtime\fR is set equal to the current time.
.RE
.SH RETURN VALUES
.sp
.LP
Upon successful completion, \fBmsgrcv()\fR returns a value equal to the number
of bytes actually placed into the buffer \fImtext\fR. Otherwise, \fB\(mi1\fR is
returned, no message is received, and \fBerrno\fR is set to indicate the error.
.SH ERRORS
.sp
.LP
The \fBmsgrcv()\fR function will fail if:
.sp
.ne 2
.na
\fB\fBE2BIG\fR\fR
.ad
.RS 10n
The value of \fBmtext\fR is greater than \fImsgsz\fR and
(\fImsgflg\fR\fB&MSG_NOERROR\fR) is 0.
.RE

.sp
.ne 2
.na
\fB\fBEACCES\fR\fR
.ad
.RS 10n
Operation permission is denied to the calling process.  See \fBIntro\fR(2).
.RE

.sp
.ne 2
.na
\fB\fBEIDRM\fR\fR
.ad
.RS 10n
The message queue identifier \fImsqid\fR is removed from the system.
.RE

.sp
.ne 2
.na
\fB\fBEINTR\fR\fR
.ad
.RS 10n
The \fBmsgrcv()\fR function was interrupted by a signal.
.RE

.sp
.ne 2
.na
\fB\fBEINVAL\fR\fR
.ad
.RS 10n
The \fImsqid\fR argument is not a valid message queue identifier.
.RE

.sp
.ne 2
.na
\fB\fBENOMSG\fR\fR
.ad
.RS 10n
The queue does not contain a message of the desired type and
(\fImsgflg\fR\fB&IPC_NOWAIT\fR) is non-zero.
.RE

.sp
.LP
The \fBmsgrcv()\fR function may fail if:
.sp
.ne 2
.na
\fB \fBEFAULT\fR\fR
.ad
.RS 11n
The \fImsgp\fR argument points to an illegal address.
.RE

.SH USAGE
.sp
.LP
The value passed as the \fImsgp\fR argument should be converted to type \fBvoid
*\fR.
.SH ATTRIBUTES
.sp
.LP
See \fBattributes\fR(7) for descriptions of the following attributes:
.sp

.sp
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE	ATTRIBUTE VALUE
_
Interface Stability	Standard
.TE

.SH SEE ALSO
.sp
.LP
.BR Intro (2),
.BR msgctl (2),
.BR msgget (2),
.BR msgsnd (2),
.BR sigaction (2),
.BR attributes (7),
.BR standards (7)
